This invention relates in general to color television picture tubes and in particular, to a system for implosion protecting such tubes. Conventionally, a color television picture tube has a glass bulb including a funnel, a flanged faceplate sealed to the flared end of the funnel, and an electron gun assembly mounted in the funnel neck for providing a source of cathode rays. The faceplate has a concave inner surface on which is deposited an electron-excitable phosphor screen. After the faceplate is sealed to the funnel, the glass bulb is evacuated and as a result, several tons of atmospheric pressure is exerted against the external surface of the faceplate. A glass bulb of this type is subject to implosion. The term "implosion" is defined by Underwriters Laboratory Incorporated as "rapid and sudden inward bursting of a high-vacuum glass envelope." It is of the utmost importance in the interest of safety to prevent the faceplate from violently shattering should it be struck for example by a heavy missile, for when a bulb implodes fragments of glass may fly forwardly from the tube into the viewing area.
Three basic approaches for implosion protecting color cathode ray tubes (CRT's) have evolved. These three approaches employ different principles of operation. One approach is implemented in systems referred to as "rimbond" systems. The rimbond system has a scalloped metal frame which surrounds the flange found on every conventional faceplate. The gap between the frame and the faceplate flange is filled with a cement -- typically an epoxy cement. In a rimbond system, the frame is not under tension. The cement holds in position the pieces of glass of a shattered faceplate long enough for air to enter the tube through the cracks formed so that pressure builds up in the tube relatively slowly. This prevents unacceptable amounts of glass from being projected forwardly from the tube although the tube may still collapse. Patents illustrating such rimbond systems are U.S. Pat. Nos. 3,485,407; 3,558,818; 3,412,203 and 3,835,250. A major drawback to such rimbond systems has been the large amounts of very costly epoxy cement needed to adhere the metal frame to the faceplate.
A second basic implosion protection approach is termed the "tension band" approach. Systems implementing this approach comprise a strap or band which is placed around the faceplate flange and put under very high tensile force. Numerous patents have been issued on various aspects of tension band systems. See U.S. Pat. Nos. 3,818,557; 3,777,057, 3,845,530; and 3,890,464. The tension band systems, however, also have several drawbacks. When the tension band is tightened about the faceplate flange, it is very likely that the glass will be scratched as the band moves across it during the tightening process. This creates flaws at the location of the scratches, increasing the possibility of cracks forming there during implosion. Also, the distribution of forces applied to the faceplate flange by the band is irregular. Specifically, the forces applied at the corners by the band are much greater than the forces applied at the sides of the faceplate flange.
The third approach is to bond a transparent protective shield over the front surface of the faceplate. Systems following this approach are commonly termed "bonded panel" systems. The bonded panel systems have no pertinence to this invention and therefore will not be discussed further.
A U.S. Pat. No. 2,222,197 to Engels discloses a CRT in which the CRT envelope comprises a curved flangeless faceplate insert in an expanded open end of a cooperating funnel. A band allegedly providing implosion protection surrounds the funnel near the open end thereof in a plane intersecting the faceplate which is ensconsed within the funnel mouth. The Engels system is quite different from the present system. No frame of any sort is provided. The implosion band environs the funnel rather than the faceplate peripheral edge. A comparison of the Engels system and the present system will reveal other important differences also.
This invention is believed to be most useful when applied to a tube having a flangeless faceplate. This tube is disclosed in U.S. Pat. No. 3,894,260, issued to the assignee of this application. The tube has a flangeless, curved glass faceplate, a concave inner surface of which receives a phosphor screen. The funnel portion of this unique tube has a convex seal land which matches and mates with the curvature of the concave inner surface of the faceplate. Since the faceplate is flangeless, and the sealing interface between the funnel and faceplate is curved rather than planar as in conventional tubes, the relevant conventional structures for implosion protection cannot be used with this tube.